This invention relates to converter circuits and more particularly to a monolithic integrated circuit for producing a pair of logarithmic differential output currents in response to an applied linear voltage.
The circuit of the present invention may be used in audio circuits to linearly vary the gain, in decibels, versus percent rotation of the linear potentiometer. A direct application of such a circuit is the utilization thereof in a transceiver system for simultaneously increasing the gain of one channel (either transmit or receive) and decreasing the gain in the other channel to maintain a constant system gain.
One particular application of the circuit of the invention is in speaker phone circuits for differentially controlling the gains of the transmit and receive channels. In some contemporary speaker phone systems the close proximity of the speaker to the microphone has caused oscillation problems. In order to overcome oscillation, these prior art systems have required a total system gain equal to some predetermined constant value. To maintain this constant value sensing circuitry is provided to determine in what mode (transmit or receive) the system is in so that the gain of that particular channel is maximized simultaneously with minimizing the gain of the other channel. This in effect has prevented these prior art systems from being a true transceiver circuit, i.e., a user cannot simultaneously talk and listen using the prior art system.
The circuit of the present invention may be utilized to overcome the shortcomings of the prior art systems. The differential output currents provided by the present invention may be utilized to automatically and differentially vary the gain of the transmit and receive channels in response to voice control. Therefore, the circuit of the present invention can provide instantaneous two-way communication not presently realizable in some contemporary speaker phone systems.
It is further contemplated that the circuit of the present invention may be utilized in similar audio circuits such as citizen band radios. The hand held mike presently required could then be replaced to provide hands off two-way radio communication.